Throughout a lifetime, bones and joints become damaged and worn through normal use, disease, and traumatic events. Arthritis is a leading cause of joint damage that leads to cartilage degradation, pain, swelling, stiffness, and bone loss overtime. Arthritis can also cause the muscles articulating the joints to lose strength and become very painful.
If the pain associated with the dysfunctional joint is not alleviated by less-invasive therapies, a joint arthroplasty procedure is considered as a treatment. Joint arthroplasty is an orthopedic procedure in which an arthritic or dysfunctional joint surface is replaced with an orthopedic prosthesis.
The accurate placement and alignment of an implant is a large factor in determining the success of joint arthroplasty. A slight misalignment may result in poor wear characteristics, reduced functionality, poor clinical outcomes, and decreased prosthetic longevity.
In order to achieve accurate implant placement and alignment, the cutting tool must be accurately positioned relative to the bone prior to making any bone cuts and/or modifications. In some methods, a cutting jig may be used to accurately position and orient a cutting tool such as a saw, drill, or reamer. In other methods, the cuts may be made using a computer-assist device (e.g., a surgical robot) that controls a saw, cutter, or reamer. When a computer-assist device is used to make the cuts, the bone's position and orientation (POSE) must be known precisely in three-dimensional space (and hence relative to the computer-assist device) to ensure that the cuts and/or modifications are made in the correct location. Several methods to determine the POSE of a bone relative to a computer-assist device are known in the art such as the registration methods described in U.S. Pat. Nos. 6,033,415 and 5,951,475.
However, bone motions during the process of cutting and implant replacement may generate cutting inaccuracies during the bone surgery if the bone is fixed with respect to the computer-assist device and not tracked in 6-DOF by a tracking system where the registration may otherwise be updated as the bone moves. Should a sufficient amount of bone motion occur, it is then necessary to immediately stop the cutting operation and restart the cutting procedure after re-registering the position of the bone with respect to the computer-assist device. Additionally, if a 6-DOF tracking system is used to update the registration as the bone moves, the tracking device (e.g., a tracking array of an optical tracking system) attached to the bone may still move relative to the bone after the initial registration, in which case the 6-DOF tracking is no longer valid and bone re-registration is still required.
In order to facilitate the process of restoring the registration after bone motion occurs, a system and method using recovery markers placed on the bone may be employed as described in U.S. Pat. No. 6,430,434. These recovery markers can be used to quickly re-register the bone by re-digitizing the location of the recovery markers.
In order to accurately resolve six degrees of freedom (6-DOF) of bone motion to recover the registration, the recovery markers must be placed a certain distance apart. In general, as the distance between the recovery markers increases, the accuracy in resolving 6-DOF also increases. Thus, there is typically a lower limit distance, either pre-determined experimentally, mathematically, or via simulations, in which the recovery markers must be distanced apart to accurately resolve all 6-DOF. This lower-limit distance is referred to herein as the “pre-determined distance”. Currently, the installation of the recovery markers is accomplished by a user installing a first recovery marker on the bone using a standard surgical drill. After the first recovery marker is installed, the user is either guessing where to place a subsequent recovery marker in the bone to satisfy the pre-determined distance, or a physical ruler is used to measure the pre-determined distance. However, this method is very time consuming and frustrating for the surgical team. In addition, there is often limited exposure of the bone, making it even more difficult to find a suitable location for any subsequent recovery marker that also satisfies the pre-determined distance.
Thus, there is a need for a more efficient method for determining an accurate location for a subsequent recovery marker to permit a re-registration of a bone within a desired accuracy.